High power RF amplifiers are configured using several smaller amplifiers connected by RF hybrid splitter/combiner circuits. One such RF power circuit is the Wilkinson hybrid which is composed of two transmission lines and an isolation resistor. The resistor is typically constructed with a thin film resistive element placed on an insulating beryllia (BeO) substrate. The BeO substrate acts as a dielectric which exhibits a parasitic distributive capacitance from the resistive element to ground.
In the Wilkinson hybrid, the resistor terminals are driven with common mode RF signals. Accordingly, because the distributive capacitance of the resistor is a low impedance at RF, current is shunted through the resistor to ground creating undesirable steady state power losses. Such losses reduce the efficiency of the hybrid, thereby degrading the overall performance of the RF amplifier.
There is therefore a need for an improved high power RF resistor which reduces inherent stray capacitance normally associated with the resistor when used for isolation purposes in an RF hybrid.